Iron bearing minerals in oxide form are used as raw material in direct reduction processes where a substantial portion of the iron in oxide form is reduced to elemental iron without melting using reductants consisting of gaseous hydrogen, gaseous carbon monoxide or solid carbon bearing reductants, or any combination of these reductants.
The iron bearing materials which are useful in direct reduction also contain non-iron minerals known as "gangue". The most common gangue minerals are silica (SiO.sub.2), alumina (Al.sub.2 O.sub.3), magnesia (MgO) and lime (CaO). Since the direct reduction process is a solid state reduction process, the non-iron bearing "gangue" materials remain in the direct reduced iron product which is used as raw material in the steelmaking process.
During the steelmaking process, gangue contained in the direct reduced iron is separated from the iron by melting and flotation, forming a "slag" which is removed prior to further processing of the liquid steel. The slag also contains wustite (FeO), along with sulfur and phosphorous.
After melting in the electric furnace, the crude liquid steel is separated from the slag during tapping. The crude liquid steel is refined in a ladle prior to solidification. The refining may include removal of sulfur, adjustment of carbon content, addition of manganese, silicon, and aluminum, and adjustment of temperature, after which the refined liquid steel is solidified into the desired shape through a continuous casting process.
The existing electric steelmaking process, using direct reduced iron as the primary raw material, is based on a slag chemistry with a ratio (known as the B4 ratio) of (lime+magnesia)/(silica+alumina) generally greater than 2.0. By producing this highly basic slag chemistry, retention in the slag of sulfur and phosphorous in direct reduced iron materials and scrap is facilitated and along with minimization of the wear rate of the typical magnesia (85%-90%), and carbon (10% to 15%) refractory linings used in the established steelmaking process.
Most iron bearing minerals useful for direct reduction contain a B4 ratio which is substantially lower than 2.0. As a result, the current practice in electric steelmaking process requires the addition of slag forming materials containing lime and magnesia, either blended with the iron bearing mineral during agglomeration before it is processed in the direct reduction process (most commonly into pellets) or added after the direct reduction process into the steelmaking furnace, in order to achieve a slag B4 ratio of greater than generally 2.0.
With the existing electric steelmaking process using a slag B4 ratio of generally greater than 2.0 in order to protect the sidewall of the furnace from being dissolved by the slag, the cost of converting direct reduced iron into steel increases in proportion to the quantity of silica and alumina contained in direct reduced iron, due to (1) the higher consumption of lime and dolomite (magnesia), (2) the higher slag volume, (3) the additional electricity consumption for melting the increased slag volumes, and (4) a decrease in liquid steel yield due to the iron contained in the slag in the form of wustite and/or entrained elemental iron.